1. Field of the Invention
This invention relates to organizing the flow of a fluid in a vertical heat exchanger to prevent the formation of a stagnant volume therein and also to direct sludge forming contaminants occurring in the fluid toward predetermined sites for withdrawal from the heat exchanger. More particularly, the present invention is useful in a steam generator of the type in which a multiplicity of heating tubes extend from a tube plate and wherein a rotational flow is induced in the steam generating fluid by introducing the fluid into a bottom central portion of the generator just above the tube plate. The rotational flow washes radially outwardly across the surface of the tube plate to carry salts and/or other contaminants capable of forming sludge to predetermined collection and discharge sites at the periphery of the tube plate.
2. Description of the Prior Art
As is known in the art, a steam generator is a form of heat exchanger used in power plants, particularly nuclear power plants, to produce steam which is used to drive a turbine-generator for the production of electric power. A steam generator of this type includes a bundle of vertically orientated tubes inside a cylindrical shell. In certain steam generators, the tubes are bent to a U-shaped form and fastened in inverted positions to a tube sheet in a lower part of a shell. In other types of steam generators, also called "once through", the tubes are straight and fastened to lower and upper tube sheets located inside the shell. In both types of steam generators, the primary coolant from the nuclear reactor flows inside the vertically orientated tubes in order to heat a fluid medium used for generating steam which flows upwardly in the spaces between the tubes. The tubes in the bundle-formation are relatively long to allow a needed residence time for the heat exchange process. As the fluid rises in the steam generator along the outside surfaces of the bundle of tubes, the fluid is converted to gaseous steam to an ever increasing extent so that after leaving the heat exchanger the steam is separated from the liquid and essentially only steam is drawn off at the top portion of the steam generator in order to drive a turbine generator.
In the liquid-to-gas conversion process the upward volumetric flow rate is ever increasing with a concurrent decreasing density. Under these operating conditions and at the operating pressures of the steam generator, the average concentration of the salt content in the liquid volume increases with time because of the above-mentioned evaporation process, chemical factors, thermal factors, and still other factors. When the concentration of the salt contaminate becomes too great, the salt begins to precipitate out of solution to form sludge deposits.
An equilibrium salt concentration can be achieved in the steam generator water with the organization of a partial blowdown flow and an excess clear feedwater uptake. Nevertheless, the proper selection of blowdown rates does not prevent the sludge collection in a conventional steam generator with turbulent intake at the bottom of the heat exchanger. This is because the low upward flow velocity together with uncontrolled organization of flow channels due to the conversion of liquid to steam contribute to the organization of stagnate fluid volumes. At these stagnate volumes, the sedimentation of the salt, both in the form of ions and in the form of solid particles, can be seen as the intermediate cause for sludge formation. The stagnation sites are prevalent at the cooler flow spaces between the tubes in the lower central part of a steam generator.
In a typical steam generator of the type described, an entry flow of a mixture of feed water and steam generator water is directed into the lower portion of a steam generator through circumferential opening just above the tube sheet and communicating with a downcomer flow space between inner and outer shell members of the generator. As the water mixture enters the tube interspace from the downcomer, the immediate effect by the heat exchange process is an induced upward flow of the fluid, turning the initial horizontal flow of water from the circumferential opening upward, causing channels with elevated velocity to form in accordance with local heat-up rates and isolating the colder parts of the heat exchanger, whereby a stagnation to the fluid flow is further induced. A change in the selection of blowdown uptake location also cannot solve the problem because of the self reorganization of flows under uncontrolled normal operation flow conditions.
The stagnate volume allows the sludge to accumulate on the tube sheet, rise along the tubes and become situated in colder central volumes. Under these conditions, local corrosion occurs. With the ever increasing build-up of sludge, there occurs a need for more frequent down time for maintenance, particularly clean-up operations. As a consequence, more limited and costly restrictions are usually imposed on the quality of feed water that can be used in the steam generator.